The same metabolite, PC(38:4), is also associated with Trans B cell expression of PLD1 (Phospholipase D1) (Figure 3D), which catalyzes the hydrolysis of phosphatidylcholine (PC) to produce phosphatidic acid (PA) [58] and plays an important role in inflammation, bone demineralization and osteoclast differentiation in the context of rheumatoid arthritis [59, 60], suggesting similar but unexplored roles in OA. This evidence concerns the gene PLD1 and rheumatoid arthritis.